#include<iostream>
#include<string>

using namespace std;

char myc[100000]={0};


int main(){
    long long n;
    string s;
    cin>>n>>s;

    for(int i=0;i<s.size();){
        char ch=s[i];
        myc[ch-'a']=s[i];
        while(s.find(ch)!=string::npos){
            s.erase(s.find(ch),1);
        }
    }

    for(int i=0;i<26;i++){
        if(myc[i]>='a'&&myc[i]<='z')
            cout<<myc[i];

    }

    return 0;
}
